1. Field of the Invention
The present invention relates to a polyamic acid composition suitable for use in forming a polyimide film pattern by means of exposure and development, and to a polyamic acid composition which can be imidized at low temperatures.
2. Description of the Related Art
Polyimide resin has hitherto been used as material of liquid-crystal orientation film of a liquid crystal display and as material of insulating films and protective films of a semiconductor device.
In a semiconductor device, for example, a protective (i.e., passivation) film is formed on the semiconductor substrate, to protect the elements formed in the substrate, from the external environment and also to enhance the reliability of the semiconductor device. Used widely as material of the protective film is polyimide resin, which has high radiation resistance, high heat resistance, and excellent electrical properties such as insulating property. Polyimide resin is commonly used, also as material of interlayer insulating film of a multilayerd semiconductor device.
Such a polyimide resin film can easily be formed from polyamic acid which is a precursor of the polyimide resin. More specifically, a varnish of polyamic acid is coated on the surface of a substrate and dried, thus forming a film. The film, thus formed, is heated, thereby cyclizing the polyamic acid. As a result, the film is imidized into a polyimide resin film. Thereafter, the photolithography of the ordinary type is performed on the resin film, thereby forming a polyimide film pattern. This method of forming a polyimide film pattern can be performed with a stabilizing treatment at a relatively low temperature, and is therefore employed widely in the manufacture of semiconductor devices.
In recent years, attempts have been made to use a polyamic acid composition which can be applied as photoresist for forming patterns, in order to simplify the process of manufacturing semiconductor devices. For example, Jpn. Pat. Appln. KOKAI Publication 4-218051 discloses a method using a photosensitive polyamic acid composition that contains a polyamic acid derivative which has a repeating unit represented by the following formula (A) and having at side chains organic groups having hydroxyl groups directly bonded to an aromatic ring and a photosensitizer. ##STR1## (R.sup.11 is a tetravalent organic group, R.sup.12 is a divalent organic group, and R.sup.13 and R.sup.14 are each a monovalent organic group or a hydroxyl group, at least one of R.sup.13 and R.sup.14 being an organic group having a hydroxyl group directly bonded to an aromatic ring.)
The above-mentioned polyamic acid derivative can be synthesized by two methods. The first method comprises the steps of: forming polyamic acid through reaction between tetracarboxylic dianhydride and diamine compound; and making the polyamic acid to react with an alcohol compound, an amine compound or an alkoxy compound, each compound having a hydroxyl group directly bonded to an aromatic ring, in the presence of a dehydrating agent, thereby synthesizing a polyamic acid derivative. The second method comprises the steps of: causing tetracarboxylic dianhydride to react with an alcohol compound, an amine compound or an alkoxy compound, each compound having a hydroxyl group directly bonded to an aromatic ring, thereby forming a compound; and dehydrating and condensing diamine compound and the compound formed by the above reaction, thereby synthesizing a polyamic acid derivative. In either method, however, side reaction takes place during the dehydration, inevitably causing cyclization and, subsequently, gelation of some of the polymers. The polyamic acid composition becomes less soluble to a development solution. Consequently, the polyimide film pattern formed from the aforementioned polyamic acid composition has but an insufficient resolution. With the second method, in particular, it is difficult to obtain a polyamic acid derivative having a high molecular weight. Thus, a polyimide film pattern having a high degree of polymerization cannot be formed from the polyamic acid derivative prepared by the second method.
In an liquid crystal device it is required that the liquid crystal be oriented appropriately. The method, generally employed to form a film which properly orients liquid crystal, comprises the steps of: coating a varnish of polyamic acid on the ITO transparent electrodes formed on a transparent substrate; imidizing the varnish, thereby forming a polyimide film; and rubbing the polyimide film thus formed. This method, however, includes heat treatment conducted at 300.degree. C. or more, which would discolor the color filters incorporated in a color liquid crystal display.
To prevent such discoloring of the color filters, a varnish of polyimide resin soluble in an organic solvent is coated directly on the surface of a substrate. However, the polyimide resin of this type will cause a problem when the technique of orienting liquid crystal in two different directions within each pixel, known as "dual domain technique," is applied in order to increase the view angle of the liquid crystal display. The dual domain technique is achieved by rubbing the polyimide film in one direction, forming photoresist to cover one half of each pixel, and rubbing the film in the other direction, while using the photoresist as a mask. The photoresist is formed to cover one half of every pixel, by means of exposure and development. When the photoresist is developed with a development solution, the polyimide film is immersed in the development solution which a mixture of a developing agent and an organic solvent. Made of polyimide resin soluble with an organic solvent, the film is damaged, ultimately reducing performance of the liquid crystal display.
As described above, the polyimide film pattern made from the conventional photosensitive polyamic acid composition has but an insufficient resolution. Furthermore, in the manufacture of a liquid crystal display or the like, the heat applied on the polyamic acid composition, imidizing the polyamic acid and forming a polyimide film to serve as a liquid-crystal orientation film, would inevitably affect the display characteristics of the liquid crystal display.